Titanium dioxide, which is usually used as photocatalyst removing environmental pollutants, pigment materials, additives for plastic product or optical multi-coating reagent, has two phases of crystalline structure, that is anatase and rutile. Titanium dioxide with anatase phase has been used as a photocatalyst for photodecomposition of acetone, phenol or trichloro ethylene, oxidation system of nitric oxide such as nitrogen mono-oxide and nitrogen dioxide and conversion system of solar energy because of its high photo-activity. Titanium dioxide with rutile phase has been widely used for white pigment materials because of its good scattering effect that protects the ultraviolet light. It has also been used for optical coating, beam splitter and anti-reflection coating since it has a high dielectric constant and refractive index, a good oil adsorption ability and tinting power, and chemical stability, even under strongly acidic or basic conditions. Titanium dioxide shows different electrical characteristics according to oxygen partial pressure since it has wide chemical stability and non-stoichiometric phase region. Because of this, it can also be used for a humidity sensor and a high-temperature oxygen sensor, and the field of its use has become wide.
Generally, titanium dioxide powders are fabricated by a chloride process, which is a gas phase process, or by a sulfate process, which is a liquid phase process.
In the chloride process, which was industrialized by Du Pont in USA in 1956, titanium tetrachloride, vigorously reacting with moisture in the air and undergoing hydrolysis, is used as a starting material and the reaction temperature needs to be higher than 1,000.degree. C. Also, this method requires extra protection devices because of the corrosive HCl or Cl.sub.2 gas by-produced in the process, leading to higher production costs. Because titanium dioxide powders produced by the chloride process are fine but rough, additive equipment for giving external electric fields or controlling reactant mixing ratios are required to control the particle shape and the particle size of titanium dioxide powders.
In the sulfate process, which was industrialized by Titan company in Norway in 1916, titanium sulfate (TiSO.sub.4) is conventionally hydrolyzed at temperatures higher than 95.degree. C., calcined at 800-1,000.degree. C. and then pulverized to produce titanium dioxide powders. During these calcination and pulverization processes, impurities are introduced causing the quality of the final titanium dioxide powder to be low.
However, compared with gas phase process, the liquid phase process represented by the sulfate process needs a milder temperature condition and makes it possible to produce titanium dioxide in a large amount, thus there have been some other reports about an improved liquid phase process or new liquid phase process to fabricate crystalline titanium dioxide powder using titanium tetrachloride, the starting material in the chloride process.
Russia patent SU-1,398,321 shows a new liquid phase process, in which an adequate amount of anatase phase titaniferous seed was added into titanium tetrachloride solution, hydrolyzed to precipitate titanium dioxide powders by heating and the precipitated titanium dioxide was fabricated by an additional process such as a high temperature treatment. This process is simple but requires additive high temperature treatment of 600-650.degree. C. to obtain anatase phase titanium dioxide, and a much higher temperature treatment to obtain rutile phase titanium dioxide.
In addition, in JP 9-124,320, gel was formed by adding water to titanium tetrachloride dissolved in alcohol such as butanol, together with one of various kinds of acetate, carbonate, oxalate and citrate containing alkali metals or alkali earth metals. Then, the obtained gel was treated with high temperature and titanium dioxide was fabricated. The physical properties of titanium dioxide powders produced by this method are good, but the process requires expensive additives such as organic acids and needs a high temperature treatment to remove added organic acids after gel formation.
Another process like sol-gel method and hydrothermal synthesis has been developed to control the titanium dioxide powder characteristics such as particle shape, particle size and distribution of the particle size. Metal alkoxide is usually used to fabricate spherically shaped titanium dioxide powders with a uniform size on a laboratory scale and this sol-gel method using alkoxide produces fine spherically shaped powders with a uniform, size smaller than 1.0 .mu.m. However, tight control of the reaction conditions is required since alkoxide is intensely hydrolyzed in air. Furthermore, the high price of the alkoxide limits its commercialization. The hydrothermal synthesis using an autoclave under high temperature and pressure conditions produces high quality powders but a continuous process has been impossible up to now.
The present inventors have successfully developed a new titanium dioxide powder fabrication method. In the method, it is possible to prepare titanium dioxide powder with good characteristics such as particle shape, particle size and distribution of the particle size, reproducibly and continuously. In addition, it is easy to control the mixture ratio of rutile and anatase phase of the titanium dioxide crystalline.